The Baxandall parallel class-D oscillator "squegs" in real life if you build it with a high value inductance in the current feed and bipolar switches. An LTSpice circuit using the Gummel-Poon model for the bipolar transistors doesn't show this - it settles down to a stable sinusoidal oscillation within less than a hundred cycles. The obvious effect of using a high value inductor is that the current through the inductor over-shoots dramatically at start-up and actually reverses polarity as it recovers, so the bipolar transistors briefly run inverted (emitter and collector diffusions swap roles). In real life this clearly makes the circuit behave very oddly, but the Gummel- Poon model equally clearly doesn't capture this particular oddness. VBIC might do better, but it doesn't with the model parameters that I bodged out of Gummel-Poon parameters. Somebody who had a better - some - understanding of the VBIC parameters might well be able to do better. Version 4 SHEET 1 3272 1144 WIRE -160 -160 -464 -160 WIRE 80 -160 -160 -160 WIRE 368 -160 80 -160 WIRE 80 -128 80 -160 WIRE -160 -64 -160 -160 WIRE 368 -64 368 -160 WIRE -64 -16 -96 -16 WIRE 80 -16 80 -48 WIRE 80 -16 16 -16 WIRE 112 -16 80 -16 WIRE 176 -16 112 -16 WIRE 304 -16 256 -16 WIRE -160 96 -160 32 WIRE -80 96 -160 96 WIRE 0 96 -80 96 WIRE 288 96 64 96 WIRE 368 96 368 32 WIRE 368 96 288 96 WIRE -160 192 -160 96 WIRE -112 192 -160 192 WIRE 80 192 -32 192 WIRE 208 192 80 192 WIRE 368 192 368 96 WIRE 368 192 288 192 WIRE -224 272 -288 272 WIRE 16 272 -144 272 WIRE 80 272 80 192 WIRE 80 272 16 272 WIRE -464 336 -464 -160 WIRE -128 416 -160 416 WIRE -32 416 -128 416 WIRE 192 416 48 416 WIRE 224 416 192 416 WIRE 384 416 304 416 WIRE 400 416 384 416 WIRE 112 464 112 -16 WIRE -160 560 -160 416 WIRE -48 560 -160 560 WIRE 400 560 400 416 WIRE 400 560 32 560 WIRE -464 624 -464 416 WIRE -288 624 -288 272 WIRE -288 624 -464 624 WIRE 112 624 112 544 WIRE 112 624 -288 624 WIRE 192 624 192 416 WIRE 192 624 112 624 WIRE -464 656 -464 624 FLAG -464 656 0 FLAG 16 272 Vct FLAG 384 416 Vout+ FLAG -128 416 Vout- FLAG -80 96 tank- FLAG 288 96 tank+ SYMBOL ind2 -128 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 250=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.022 Cpar=3D1p SYMBOL ind2 192 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L2 SYMATTR Value 250=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.022 Cpar=3D1p SYMBOL cap 64 80 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 46 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 100n SYMBOL ind2 -240 288 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L7 SYMATTR Value 33m SYMATTR SpiceLine Ipk=3D0.03 Rser=3D80 Cpar=3D8.5p SYMATTR Type ind SYMBOL voltage -464 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=3D0.001 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL ind2 32 0 M270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L3 SYMATTR Value 1.4=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.001 Cpar=3D1pF SYMBOL ind2 272 0 M270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L4 SYMATTR Value 1.4=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.001 Cpar=3D1p SYMBOL res 96 448 R0 SYMATTR InstName R1 SYMATTR Value 3k9 SYMBOL pnp -96 32 R180 SYMATTR InstName Q3 SYMATTR Value 2N3906A SYMBOL pnp 304 32 M180 SYMATTR InstName Q4 SYMATTR Value 2N3906A SYMBOL ind2 -48 432 R270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L5 SYMATTR Value 22.5=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.004 Cpar=3D1pF SYMBOL ind2 208 432 R270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L6 SYMATTR Value 22.5=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.004 Cpar=3D1pF SYMBOL res 48 544 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res 64 -144 R0 SYMATTR InstName R3 SYMATTR Value 1k TEXT -488 720 Left 2 !.tran 0 10m 0m 10n TEXT -488 760 Left 2 !.ic V(tank-)=3D5 V(Vct)=3D4.995 V(tank+)=3D4.99 V(Vo= ut +)=3D0.0 V(Vout-)=3D-0.0 I(L3)=3D0.00 I(L1)=3D0 I(L2)=3D0 I(L4)=3D-0.0 I(L5)=3D-0.0 I(L7)=3D-0.0033 TEXT -480 800 Left 2 !K1 L1 L2 L3 L4 L5 L6 0.99 TEXT -488 840 Left 2 !.model 2N3906A PNP(ibei=3D2.22716E-18 xii=3D3 Ea=3D1.11 Vef=3D16.8 \n+ Iben=3D7.558f Nen=3D1.536 Ikf=3D.293583 Xin=3D1.= 5 Ver=3D50 ibcn=3D529.3E-18 \n+ Ncn=3D15.51 Ikr=3D10. Rcx=3D.8508 Cjc=3D10.1= 3p Mc=3D. 6993 pc=3D1.006 Fc=3D.5 \n+ Cje=3D10.39p Me=3D.6931 pe=3D.9937 Tr=3D10n Tf= =3D181.2p Itf=3D4.881m rth=3D0. cth=3D0.\n+ Xtf=3D.7939 Vtf=3D10 Rbx=3D10)\n+ Level= =3D9 -- Bill Sloman, Nijmegen
Baxandall class-D oscillator squegging and the VBIC model
Started by ●January 13, 2012
Reply by ●January 14, 20122012-01-14
On Fri, 13 Jan 2012 17:44:04 -0800, Bill Sloman wrote:> The Baxandall parallel class-D oscillator "squegs" in real life if you > build it with a high value inductance in the current feed and bipolar > switches. An LTSpice circuit using the Gummel-Poon model for the bipolar > transistors doesn't show this - it settles down to a stable sinusoidal > oscillation within less than a hundred cycles. > > The obvious effect of using a high value inductor is that the current > through the inductor over-shoots dramatically at start-up and actually > reverses polarity as it recovers, so the bipolar transistors briefly run > inverted (emitter and collector diffusions swap roles). In real life > this clearly makes the circuit behave very oddly, but the Gummel- Poon > model equally clearly doesn't capture this particular oddness. > > VBIC might do better, but it doesn't with the model parameters that I > bodged out of Gummel-Poon parameters. Somebody who had a better - some - > understanding of the VBIC parameters might well be able to do better. >(LTSpice _schematic_ (Phil) snipped) Since it's balanced, why do you need a monster inductor there? Why not use a resistor, or put a resistor in series with the L to dampen the squegging? -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●January 14, 20122012-01-14
On Fri, 13 Jan 2012 17:44:04 -0800 (PST), Bill Sloman <bill.sloman@ieee.org> wrote:>The Baxandall parallel class-D oscillator "squegs" in real life if you >build it with a high value inductance in the current feed and bipolar >switches. An LTSpice circuit using the Gummel-Poon model for the >bipolar transistors doesn't show this - it settles down to a stable >sinusoidal oscillation within less than a hundred cycles. > >The obvious effect of using a high value inductor is that the current >through the inductor over-shoots dramatically at start-up and actually >reverses polarity as it recovers, so the bipolar transistors briefly >run inverted (emitter and collector diffusions swap roles). In real >life this clearly makes the circuit behave very oddly, but the Gummel- >Poon model equally clearly doesn't capture this particular oddness. > >VBIC might do better, but it doesn't with the model parameters that >I bodged out of Gummel-Poon parameters. Somebody who had a better - >some - understanding of the VBIC parameters might well be able to do >better. >It seems to wobble a bit on startup, but doesn't actually squegg even with a full henry as the feed inductor. Oscillators sure simulate slowly. John
Reply by ●January 14, 20122012-01-14
On Jan 14, 5:36=A0am, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Fri, 13 Jan 2012 17:44:04 -0800 (PST), Bill Sloman > > > > > > > > > > <bill.slo...@ieee.org> wrote: > >The Baxandall parallel class-D oscillator "squegs" in real life if you > >build it with a high value inductance in the current feed and bipolar > >switches. An LTSpice circuit using the Gummel-Poon model for the > >bipolar transistors doesn't show this - it settles down to a stable > >sinusoidal oscillation within less than a hundred cycles. > > >The obvious effect of using a high value inductor is that the current > >through the inductor over-shoots dramatically at start-up and actually > >reverses polarity as it recovers, so the bipolar transistors briefly > >run inverted (emitter and collector diffusions swap roles). In real > >life this clearly makes the circuit =A0behave very oddly, but the Gummel=-> >Poon model equally clearly doesn't capture this particular oddness. > > >VBIC might do better, but it doesn't with the model parameters that > >I =A0bodged out of Gummel-Poon parameters. Somebody who had a better - > >some - understanding of the VBIC parameters might well be able to do > >better. > > It seems to wobble a bit on startup, but doesn't actually squeg even > with a full henry as the feed inductor.That's what I'm complaining about. A real circuit would almost certainly squeg. -- Bill Sloman, Nijmegen
Reply by ●January 14, 20122012-01-14
On Jan 14, 5:36=A0am, Tim Wescott <t...@seemywebsite.com> wrote:> On Fri, 13 Jan 2012 17:44:04 -0800, Bill Sloman wrote: > > The Baxandall parallel class-D oscillator "squegs" in real life if you > > build it with a high value inductance in the current feed and bipolar > > switches. An LTSpice circuit using the Gummel-Poon model for the bipola=r> > transistors doesn't show this - it settles down to a stable sinusoidal > > oscillation within less than a hundred cycles. > > > The obvious effect of using a high value inductor is that the current > > through the inductor over-shoots dramatically at start-up and actually > > reverses polarity as it recovers, so the bipolar transistors briefly ru=n> > inverted (emitter and collector diffusions swap roles). In real life > > this clearly makes the circuit =A0behave very oddly, but the Gummel- Po=on> > model equally clearly doesn't capture this particular oddness. > > > VBIC might do better, but it doesn't with the model parameters that I > > bodged out of Gummel-Poon parameters. Somebody who had a better - some =-> > understanding of the VBIC parameters might well be able to do better. > > (LTSpice _schematic_ (Phil) snipped) > > Since it's balanced, why do you need a monster inductor there? =A0Why not > use a resistor, or put a resistor in series with the L to dampen the > squegging?Using a big inductor reduces the level of odd harmonics (3rd, 5th etc) in the sine wave generated. Using an inductor at all is the kern of the Class-D oscillator http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf Adding more resistance in series with the inductor makes the circuit less efficient. Jim William's versions get over 90% efficiency - Linear Technology application notes AN45, AN49, AN51, AN55, AN61, AN65. If it were big enough, it might damp the overshoot on start-up enough to prevent the initial reversal of current in the inductor, which might - in turn - stop the squegging from starting. -- Bill Sloman, Nijmegen
Reply by ●January 14, 20122012-01-14
Bill Sloman <bill.sloman@ieee.org> writes:> On Jan 14, 5:36 am, John Larkin > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Fri, 13 Jan 2012 17:44:04 -0800 (PST), Bill Sloman >> >> >> >> >> >> >> >> >> >> <bill.slo...@ieee.org> wrote: >> >The Baxandall parallel class-D oscillator "squegs" in real life if you >> >build it with a high value inductance in the current feed and bipolar >> >switches. An LTSpice circuit using the Gummel-Poon model for the >> >bipolar transistors doesn't show this - it settles down to a stable >> >sinusoidal oscillation within less than a hundred cycles. >> >> >The obvious effect of using a high value inductor is that the current >> >through the inductor over-shoots dramatically at start-up and actually >> >reverses polarity as it recovers, so the bipolar transistors briefly >> >run inverted (emitter and collector diffusions swap roles). In real >> >life this clearly makes the circuit behave very oddly, but the Gummel- >> >Poon model equally clearly doesn't capture this particular oddness. >> >> >VBIC might do better, but it doesn't with the model parameters that >> >I bodged out of Gummel-Poon parameters. Somebody who had a better - >> >some - understanding of the VBIC parameters might well be able to do >> >better. >> >> It seems to wobble a bit on startup, but doesn't actually squeg even >> with a full henry as the feed inductor. > > That's what I'm complaining about. A real circuit would almost > certainly squeg.Well there's one way to find out isn't there? :) -- John Devereux
Reply by ●January 14, 20122012-01-14
On Jan 14, 11:09=A0am, John Devereux <j...@devereux.me.uk> wrote:> BillSloman<bill.slo...@ieee.org> writes: > > On Jan 14, 5:36=A0am, John Larkin > > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> On Fri, 13 Jan 2012 17:44:04 -0800 (PST),BillSloman > > >> <bill.slo...@ieee.org> wrote: > >> >The Baxandall parallel class-D oscillator "squegs" in real life if yo=u> >> >build it with a high value inductance in the current feed and bipolar > >> >switches. An LTSpice circuit using the Gummel-Poon model for the > >> >bipolar transistors doesn't show this - it settles down to a stable > >> >sinusoidal oscillation within less than a hundred cycles. > > >> >The obvious effect of using a high value inductor is that the current > >> >through the inductor over-shoots dramatically at start-up and actuall=y> >> >reverses polarity as it recovers, so the bipolar transistors briefly > >> >run inverted (emitter and collector diffusions swap roles). In real > >> >life this clearly makes the circuit =A0behave very oddly, but the Gum=mel-> >> >Poon model equally clearly doesn't capture this particular oddness. > > >> >VBIC might do better, but it doesn't with the model parameters that > >> >I =A0bodged out of Gummel-Poon parameters. Somebody who had a better =-> >> >some - understanding of the VBIC parameters might well be able to do > >> >better. > > >> It seems to wobble a bit on startup, but doesn't actually squeg even > >> with a full henry as the feed inductor. > > > That's what I'm complaining about. A real circuit would almost > > certainly squeg. > > Well there's one way to find out isn't there? :)The last one I built with an over-sized oscillator (which was the first one I ever built, more than forty years ago) certainly squegged. Building another just to prove the point would be something of a waste of time. An example with a diode to prevent the current through the inductor going the wrong way might make more sense. -- Bill Sloman, Nijmegen
Reply by ●January 14, 20122012-01-14
Bill Sloman <bill.sloman@ieee.org> writes:> On Jan 14, 11:09 am, John Devereux <j...@devereux.me.uk> wrote: >> BillSloman<bill.slo...@ieee.org> writes: >> > On Jan 14, 5:36 am, John Larkin >> > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >> On Fri, 13 Jan 2012 17:44:04 -0800 (PST),BillSloman >> >> >> <bill.slo...@ieee.org> wrote: >> >> >The Baxandall parallel class-D oscillator "squegs" in real life if you >> >> >build it with a high value inductance in the current feed and bipolar >> >> >switches. An LTSpice circuit using the Gummel-Poon model for the >> >> >bipolar transistors doesn't show this - it settles down to a stable >> >> >sinusoidal oscillation within less than a hundred cycles. >> >> >> >The obvious effect of using a high value inductor is that the current >> >> >through the inductor over-shoots dramatically at start-up and actually >> >> >reverses polarity as it recovers, so the bipolar transistors briefly >> >> >run inverted (emitter and collector diffusions swap roles). In real >> >> >life this clearly makes the circuit behave very oddly, but the Gummel- >> >> >Poon model equally clearly doesn't capture this particular oddness. >> >> >> >VBIC might do better, but it doesn't with the model parameters that >> >> >I bodged out of Gummel-Poon parameters. Somebody who had a better - >> >> >some - understanding of the VBIC parameters might well be able to do >> >> >better. >> >> >> It seems to wobble a bit on startup, but doesn't actually squeg even >> >> with a full henry as the feed inductor. >> >> > That's what I'm complaining about. A real circuit would almost >> > certainly squeg. >> >> Well there's one way to find out isn't there? :) > > The last one I built with an over-sized oscillator (which was the > first one I ever built, more than forty years ago) certainly squegged. > Building another just to prove the point would be something of a waste > of time. An example with a diode to prevent the current through the > inductor going the wrong way might make more sense.Well of course the next step is to fix the squegging. But if it does not show up in the simulation, then the simulation is not going to be much use for fixing it! -- John Devereux
Reply by ●January 14, 20122012-01-14
On Jan 14, 3:41=A0pm, John Devereux <j...@devereux.me.uk> wrote:> BillSloman<bill.slo...@ieee.org> writes: > > On Jan 14, 11:09=A0am, John Devereux <j...@devereux.me.uk> wrote: > >> BillSloman<bill.slo...@ieee.org> writes: > >> > On Jan 14, 5:36=A0am, John Larkin > >> > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> >> On Fri, 13 Jan 2012 17:44:04 -0800 (PST),BillSloman > > >> >> <bill.slo...@ieee.org> wrote: > >> >> >The Baxandall parallel class-D oscillator "squegs" in real life if=you> >> >> >build it with a high value inductance in the current feed and bipo=lar> >> >> >switches. An LTSpice circuit using the Gummel-Poon model for the > >> >> >bipolar transistors doesn't show this - it settles down to a stabl=e> >> >> >sinusoidal oscillation within less than a hundred cycles. > > >> >> >The obvious effect of using a high value inductor is that the curr=ent> >> >> >through the inductor over-shoots dramatically at start-up and actu=ally> >> >> >reverses polarity as it recovers, so the bipolar transistors brief=ly> >> >> >run inverted (emitter and collector diffusions swap roles). In rea=l> >> >> >life this clearly makes the circuit =A0behave very oddly, but the =Gummel-> >> >> >Poon model equally clearly doesn't capture this particular oddness=.> > >> >> >VBIC might do better, but it doesn't with the model parameters tha=t> >> >> >I =A0bodged out of Gummel-Poon parameters. Somebody who had a bett=er -> >> >> >some - understanding of the VBIC parameters might well be able to =do> >> >> >better. > > >> >> It seems to wobble a bit on startup, but doesn't actually squeg eve=n> >> >> with a full henry as the feed inductor. > > >> > That's what I'm complaining about. A real circuit would almost > >> > certainly squeg. > > >> Well there's one way to find out isn't there? :) > > > The last one I built with an over-sized oscillator (which was the > > first one I ever built, more than forty years ago) certainly squegged. > > Building another just to prove the point would be something of a waste > > of time. An example with a diode to prevent the current through the > > inductor going the wrong way might make more sense. > > Well of course the next step is to fix the squegging. But if it does not > show up in the simulation, then the simulation is not going to be much > use for fixing it!If the problem is the current going the wrong way through the inductor - which is what the simulations seem to suggest - then one extra diode might well fix it. If this were to work, I'd have found something - thanks to the simulation - which Peter Baxandall (amongst many others, including me and Tony Williams) had missed. If it were that simple, someone should have found it out earlier, so it probably won't work. Version 4 SHEET 1 3272 1144 WIRE -160 -160 -464 -160 WIRE 80 -160 -160 -160 WIRE 368 -160 80 -160 WIRE 80 -128 80 -160 WIRE -160 -64 -160 -160 WIRE 368 -64 368 -160 WIRE -64 -16 -96 -16 WIRE 80 -16 80 -48 WIRE 80 -16 16 -16 WIRE 112 -16 80 -16 WIRE 176 -16 112 -16 WIRE 304 -16 256 -16 WIRE -160 96 -160 32 WIRE -80 96 -160 96 WIRE 0 96 -80 96 WIRE 288 96 64 96 WIRE 368 96 368 32 WIRE 368 96 288 96 WIRE -160 192 -160 96 WIRE -112 192 -160 192 WIRE 80 192 -32 192 WIRE 208 192 80 192 WIRE 368 192 368 96 WIRE 368 192 288 192 WIRE -224 272 -304 272 WIRE 16 272 -144 272 WIRE 80 272 80 192 WIRE 80 272 16 272 WIRE -464 336 -464 -160 WIRE -128 416 -160 416 WIRE -32 416 -128 416 WIRE 192 416 48 416 WIRE 224 416 192 416 WIRE 384 416 304 416 WIRE 400 416 384 416 WIRE 112 464 112 -16 WIRE -304 496 -304 272 WIRE -160 560 -160 416 WIRE -48 560 -160 560 WIRE 400 560 400 416 WIRE 400 560 32 560 WIRE -464 624 -464 416 WIRE -304 624 -304 560 WIRE -304 624 -464 624 WIRE 112 624 112 544 WIRE 112 624 -304 624 WIRE 192 624 192 416 WIRE 192 624 112 624 WIRE -464 656 -464 624 FLAG -464 656 0 FLAG 16 272 Vct FLAG 384 416 Vout+ FLAG -128 416 Vout- FLAG -80 96 tank- FLAG 288 96 tank+ SYMBOL ind2 -128 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 250=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.022 Cpar=3D1p SYMBOL ind2 192 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L2 SYMATTR Value 250=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.022 Cpar=3D1p SYMBOL cap 64 80 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 46 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 100n SYMBOL ind2 -240 288 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L7 SYMATTR Value 33m SYMATTR SpiceLine Ipk=3D0.03 Rser=3D80 Cpar=3D8.5p SYMATTR Type ind SYMBOL voltage -464 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=3D0.001 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL ind2 32 0 M270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L3 SYMATTR Value 1.4=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.001 Cpar=3D1pF SYMBOL ind2 272 0 M270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L4 SYMATTR Value 1.4=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.001 Cpar=3D1p SYMBOL res 96 448 R0 SYMATTR InstName R1 SYMATTR Value 3k9 SYMBOL pnp -96 32 R180 SYMATTR InstName Q3 SYMATTR Value 2N3906A SYMBOL pnp 304 32 M180 SYMATTR InstName Q4 SYMATTR Value 2N3906A SYMBOL ind2 -48 432 R270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L5 SYMATTR Value 22.5=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.004 Cpar=3D1pF SYMBOL ind2 208 432 R270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L6 SYMATTR Value 22.5=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D0.004 Cpar=3D1pF SYMBOL res 48 544 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res 64 -144 R0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL schottky -320 496 R0 SYMATTR InstName D1 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode TEXT -488 720 Left 2 !.tran 0 10m 0m 10n TEXT -488 760 Left 2 !.ic V(tank-)=3D5 V(Vct)=3D4.995 V(tank+)=3D4.99 V(Vo= ut +)=3D0.0 V(Vout-)=3D-0.0 I(L3)=3D0.00 I(L1)=3D0 I(L2)=3D0 I(L4)=3D-0.0 I(L5)=3D-0.0 I(L7)=3D-0.0033 TEXT -480 800 Left 2 !K1 L1 L2 L3 L4 L5 L6 0.99 TEXT -488 840 Left 2 !.model 2N3906A PNP(ibei=3D2.22716E-18 xii=3D3 Ea=3D1.11 Vef=3D16.8 \n+ Iben=3D7.558f Nen=3D1.536 Ikf=3D.293583 Xin=3D1.= 5 Ver=3D50 ibcn=3D529.3E-18 \n+ Ncn=3D15.51 Ikr=3D10. Rcx=3D.8508 Cjc=3D10.1= 3p Mc=3D. 6993 pc=3D1.006 Fc=3D.5 \n+ Cje=3D10.39p Me=3D.6931 pe=3D.9937 Tr=3D10n Tf= =3D181.2p Itf=3D4.881m rth=3D0. cth=3D0.\n+ Xtf=3D.7939 Vtf=3D10 Rbx=3D10)\n+ Level= =3D9
Reply by ●January 14, 20122012-01-14
On Sat, 14 Jan 2012 01:56:59 -0800, Bill Sloman wrote:> On Jan 14, 5:36 am, Tim Wescott <t...@seemywebsite.com> wrote: >> On Fri, 13 Jan 2012 17:44:04 -0800, Bill Sloman wrote: >> > The Baxandall parallel class-D oscillator "squegs" in real life if >> > you build it with a high value inductance in the current feed and >> > bipolar switches. An LTSpice circuit using the Gummel-Poon model for >> > the bipolar transistors doesn't show this - it settles down to a >> > stable sinusoidal oscillation within less than a hundred cycles. >> >> > The obvious effect of using a high value inductor is that the current >> > through the inductor over-shoots dramatically at start-up and >> > actually reverses polarity as it recovers, so the bipolar transistors >> > briefly run inverted (emitter and collector diffusions swap roles). >> > In real life this clearly makes the circuit behave very oddly, but >> > the Gummel- Poon model equally clearly doesn't capture this >> > particular oddness. >> >> > VBIC might do better, but it doesn't with the model parameters that I >> > bodged out of Gummel-Poon parameters. Somebody who had a better - >> > some - understanding of the VBIC parameters might well be able to do >> > better. >> >> (LTSpice _schematic_ (Phil) snipped) >> >> Since it's balanced, why do you need a monster inductor there? Why not >> use a resistor, or put a resistor in series with the L to dampen the >> squegging? > > Using a big inductor reduces the level of odd harmonics (3rd, 5th etc) > in the sine wave generated. Using an inductor at all is the kern of the > Class-D oscillator > > http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf > > Adding more resistance in series with the inductor makes the circuit > less efficient. Jim William's versions get over 90% efficiency - Linear > Technology application notes AN45, AN49, AN51, AN55, AN61, AN65. > > If it were big enough, it might damp the overshoot on start-up enough to > prevent the initial reversal of current in the inductor, which might - > in turn - stop the squegging from starting.You might also consider putting diodes in series with the transistor collectors, to force the transistors to only work one way. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
